Various approaches have been used, for example, for passive remote sensing of the ground. Thus maps of surface brightness, temperature and emissivity have been used for investigating geological surface properties (Kahle et al. 1980, Applied Optics, 19, 2279), whereas maps of thermal inertia have been used to infer subsurface properties of soil (Price, 1977, Journal of Geophysical Research, 82, 2582).
EP-A-O 143,282; EP-A-O 420,108; and U.S. Pat. No. 5,377,126 disclose various approaches for remote determination of emissivity and temperature of an object. The determination was performed by measuring the multi-spectral electromagnetic radiation and applying various models, which provide dependence of emissivity on wavelength and temperature by solving a set of equations, which include emissivity and temperature as parameters. As such, these approaches enable to obtain the object's temperature and emissivity in various cases, including that in which these parameters vary in time.
The detection of underground structures requires penetration of the ground and is therefore accomplished by active sensing techniques, such as radar (Blake, 1993, “Ground-Penetration Radar Developed by Sweden,” International Defense Review, 3, 193; von Maydell et al, 1987, U.S. Pat. No. 4,675,677), or combined passive sensing and radar (Clark et al, SPIE—The International Society for Optical Engineering, 1942, 178).
It is acknowledged in the prior art, that emissivity of various materials may noticeably depend on wavelength of the emitted radiation and temperature of the sample.
For example, U.S. Pat. No. 4,659,234 describes a method of emissivity error correcting for radiation thermometer, where temperature is determined based on two measurements of radiated infrared energy at two closely adjoining wavelengths, and the result is corrected using one single measurement. While the method is aimed to solve a problem of determining the randomly changing value of emissivity of heated metal objects, it does not take into consideration any dependence of emissivity on the radiation wavelength, and much less—on temperature of the object. Such an approach renders the method inaccurate.
U.S. Pat. No. 5,132,922 describes an emissivity independent multi-wavelength pyrometer operating according to a method using a least-squares-based multiwavelength pyrometry technique and a theoretical function for the dependence of the radiance on the wavelength. However, in this method the emissivity/wavelength function is considered to be the same in all spectral bands, which either leads to a significant reduction in the accuracy of the determined: temperature, especially for heated non-transparent materials, or to the use of narrow-band filters. The latter poses a problem for infra-red measurements in real time. since narrow band filters provide insufficient energy for further processing.
Moreover, all the described above methods make an assumption that emissivity of an object is independent of its temperature, since none of the methods appear to suggest any way of considering this dependency for determining temperatures. Such an assumption inadvertently leads to essential errors in the temperature determination for at least non-transparent heated material.
In many branches of industry, for example in metallurgy or in manufacture of semiconductor devices samples and materials are exposed to various operations while being in the heated condition.
Determining temperature of such objects during the manufacturing process presents a very important task, which is rather difficult due to the following factors:                it is impossible to perform direct temperature measurements. so the temperature determining is usually performed via the remotely acquiring data on the infrared radiation of the sample and further processing this data,        in the mathematical processing of the remotely obtained data many parameters intrinsic to the material of interest must be used (such as its thermal emissivity, etc.), which are usually unknown and cannot be directly measured.        
Another important task is detection (usually in the dark) and recognition of various objects having different temperatures and/or emissivities. Difficulties of such a process are essentially the same as outlined above.